Ever advancing designs of integrated circuits increasingly rely on stacked silicon wafer architectures whereby conductive vertical connections across the individual wafers are essential for connecting their respective circuitries. These vertical connections are made by initially fabricating blind holes into one side of the silicon wafer. The blind holes are filled with conductive material before the wafer is thinned down on the opposite side such that the vertical conductor extends all the way between top and bottom side of the wafer. These well known Through Silicon Vias (TSV) are becoming increasingly thin in cross section, are increasingly dense arrayed and may utilize non round cross sections such as squares.
A Prior Art infrared interferometric wafer thickness sensor (WTS) is employed for measuring some of the essential geometric parameters of TSV as is depicted in Prior Art FIGS. 1A and 1B. Thereby, the silicon's transparency for infrared light is utilized to scan across the wafer height WH with a focused infrared laser beam and capture its back reflections by the same lens system that focused the laser beam LB with respect to the optical axis LA. The back reflections are interpreted to derive a scan contour SC as the focused laser beam LB is moved in scan direction SM over the TSV. During this prior art scanning process, the optical axis LA needs to be moved relative to the wafer W. Due to the bulkiness of the WTS lens system and its needs for closest proximity to the wafer bottom, an extensive peripherally positioned x-y stage system needs to be employed to provide sufficient clearance immediately beneath the wafer W across its bottom. Such peripherally positioned x-y stage system may introduce additional challenges for precise positioning and motion. In addition, such prior art WTS may preclude the positioning any well known structures for lifting and/or lowering the wafer W from and onto the wafer W fixture. Therefore, there exists a need for TSV measurement system that is sufficiently compact especially beneath the wafer W so as to not interfere with a standard compact x-y stage, the wafer W fixture and eventual additional well known structures for raising and lowering the wafer W off and onto the wafer W fixture. The present invention addresses this need.
A minimum spot width SPW of the Prior Art is limited by laser wavelength, which in turn is limited to the range at which the wafer W material is translucent as is well known in the art. At the time of this invention, the minimum spot width SPW is about 5 um for silicon wafer W. The scanning system's resolution on one hand needs a spot width SPW that is as small as possible for maximum scanning sharpness. But on the other hand, the smaller the spot width SPW the more scanning passes need to be performed for a given wafer measurement area. Also, scanning systems require precise mechanical motion for lateral measurement accuracy. Nevertheless, even precision X-Y motion stages provide only accuracy of down to about 4 um at the time of this invention, which is insufficient for TSV TCD in the single digit um range. Therefore, there exists a need for a metrology system and method that is not limited by focused laser spot width SPW and not limited by the accuracy of a mechanical motion system. The present invention addresses also this need.
Silicon has a fairly high index of refraction such that the laser entering the waver bottom WB is aligned closer to parallel. The moment it impinges on the TSV bottom VB, a portion is back reflected with phase change. Nevertheless, another portion of the laser impinging on the TSV bottom VB is forward deflected also with a phase change along the edge of the TSV bottom VB. Hence at the wafer top WT, directly entered laser and once deflected and phase changed laser impinge and are back reflected. The interaction between the oppositely phased lasers results in illumination intensity fringes that distort the measurement and scanning contour SC along the scanning distance SD as shown in Prior Art FIG. 1B. This is a key limiting issue in Prior Art back reflective laser interference metrology. Moreover, lateral fringe distortions become more dominant as TSV diameter TCD decreases and TSV aspect ratio (AR) increase. At the time of this invention, TSV depth VD with AR of only less than 15 at diameter TCD of 5 um and below are measurable with such Prior Art metrology technology. As TSV top diameters TCD are ever decreasing below 5 um, AR increases substantially above 15. For example and as is documented in the literature, the scan distance SD of an isolated TSV with 1 um TCD and AR of about 14 is about seven times CD and representative measurement width MD along that scan distance SD is about four times CD. Therefore, there exists a need for a metrology system and technology that is not adversely affected by interference fringes and that is capable of measuring AR substantially above 15 for TSV top diameters CD below 5 um. The present invention addresses this need.
As TSV AR increases, fabrication of the TSV with consistent cross section along its entire depth VD becomes increasingly challenging and needs to be more closely monitored. To fully three dimensionally characterize a TSV, location, shape and dimensions of both top diameter CD and bottom diameter BCD need to be independently measured and characterized. The TSV top diameter CD is simply accessible by well known microscopic on-top imaging and measurement techniques. Nevertheless, and in order to obtain sufficiently precise information about position of via top opening TCD and via bottom BCD, the optical axis OA of the separate top down microscopic measurement system needs to by precisely aligned with the WTS axis LA at all times. This axes OA, LA alignment is challenging to be maintained during the scanning operation where the x-y stage during rapid scan movements may introduce mass forces on the entire system structure. Therefore, there exists a need for metrology system that does not require alignment between top down optical axis OA and any bottom up laser light. The present invention addresses also this need.
Prior Art WTS requires the laser system to reach within a few microns of the wafer bottom WB. Nevertheless, with ever increasing wafer diameter and ever decreasing wafer thickness, wafer sagging becomes more and more of an issue. For example, a 300 mm diameter silicon wafer W with a common thickness of about 1 mm, sags about 200 um. This requires substantial refocusing of the bottom up laser during prior art WTS scanning. In addition, the laser focusing optics is typical several dozens of mm in diameter. This, in conjunction with the need for a small distance to the wafer bottom WB, limits wafer W periphery access to avoid collision of the laser focusing optics with the wafer fixture. Therefore, there exists a need for a TSV metrology system and method that provides for sufficient clearance to the wafer bottom such that it is unimpeded by wafer sagging and wafer fixtures. The present invention addresses also this need.